Conventional printed wiring boards designed for directly mounting electronic parts are generally used as circuit boards for timepieces, cameras and such. Use of such boards has the merit of allowing a reduction of size and thickness of the articles manufactured by using such boards.
Plastics and/or ceramics are used as materials of which these printed wiring boards are made plastic being the most widely used. Generally, printed wiring boards made of plastic can be made to precise dimensions and is stronger in mechanical shock than those made of ceramic material such as alumina. However, the thermal conductivity of plastic is very low, about 1/60 of that of alumina ceramic, and for this reason, conventional plastic boards are still unsatisfactory for practical use as a board for mounting IC's with a large scale of integration or electronic parts requiring high power.
In order to overcome this problem, various proposals, such as mentioned below, have been made on the type of printed wiring boards in which a metal plate of high thermal conductivity is adapted to the plastic board and electronic parts are directly mounted on such metal plate for better heat dispersion.
This applicant has previously disclosed a printed wiring board for mounting semiconductors and IC chips in Japanese Utility Model Laid-Open No. 10060/79. In this disclosed printed wiring board, a recess of a specified depth is provided at the part of the board where semiconductors and IC chips are to be mounted, and in the recess is further provided a through-hole of a specific configuration, with the region around said through-hole being designed to constitute a supporting region on which a thin metal sheet is seated for carrying thereon semiconductors and IC chips. This printed wiring board, however, had the problem that in the manufacture of this board, if the horizontal sectional area of the recession is increased, the space allowed for circuit arrangement on the board surface is accordingly decreased; therefore, it is necessary to enlarge the planar area of the printed wiring board itself to accommodate high-density wiring, while if the horizontal sectional area of the recess is small, the area of the supporting region decreases correspondingly so as to adversely affect adhesion between the metal sheet and the supporting region. Further, extremely high accuracy is required for working the inner peripheral surface of the recession and metal sheet for tightly fitting the metal sheet to the inner peripheral surface of the recession. It increases the cost of working and reduces productivity. There was also the problem of the reduced working life of electronic parts, as the penetration of atmospheric moisture into electrical components mounted on the metal sheet in the recession could not be prevented.
Japanese Utility Model Laid-Open No. 172970/ 81 discloses a circuit board for timepieces incorporating a laminated reinforcing material such as metal or ceramic, in which the recession for mounting LSI is formed after the reinforcement has been laminated to the circuit board. This device, however, will not prevent penetration of moisture into the LSI mounted recession through the adhesive layer bonding the board and reinforcement, thereby resulting in a reduced life of LSI.
Japanese Patent Laid-Open No. 166056/82 discloses an integrated module principally composed of a printed wiring board and logical units, that is, chips fixed thereon; in which a single-layer or multi-layer supporting base is provided with an opening into which a stepped region of a heat dissipator correctly joined to the supporting base extends, and on the side opposite from the supporting base, the logical units are directly secured on the stepped portion of the heat dissipator by a medium having high thermal conductivity. In this integrated module, however, since the stepped portion of the heat dissipator is joined to the opening of the printed wiring board, atmospheric moisture is liable to reach the logical units through the joint, so that the logical units tend to suffer early failure and have a short service life.
The present invention has for its object to provide a printed wiring board for mounting electronic parts which is free of the disadvantages of the prior art, has excellent heat dispersion characteristics moisture resistance and high reliability and which is readily adaptable to high-density wiring, and a process for producing such printed wiring board.